(1) Field of the Invention
This invention related to a pressure balanced valve and, more specifically, to such a valve which includes an axial opening through a main valve disc assembly which facilitates effective balanced operation of the valve and has been improved to prevent forces resisting closure which forces are created by fluid flow by the valve disc as it approaches the seat from interfering with effective balanced closure.
(2) Prior Art of the Invention
There have heretofore been provided large pressure balanced valves of the type disclosed in U.S. Pat. No. 3,601,157 which were primarily intended to isolate high pressure fluid flowing in a normal direction from above the valve seat. For proper operation of these valves, it was found that full, effective closure was made possible by providing a plurality of small passages through the main valve disc assembly which allowed communication between the upstream, high pressure fluid line and the top of the main valve disc assembly.
However, with increased safety and reliability requirements for some of the large power plants in which these valves might have been utilized, it became desirable to provide an alternative valve which could be closed more rapidly and was able to isolate fluid flow in either direction through the fluid line. Since the small passages were inappropriate for isolation of fluid from beneath the seat, a large bi-directional pressure balanced valve of the type disclosed in U.S. Pat. No. 3,888,280 for use in high pressure fluid lines was employed. This balanced configuration enabled fail-safe closure of the valve regardless of the direction of the flow therethrough by a plurality of springs if an opening force of a pneumatic motor assembly were no longer energized to maintain the valve in an opened position. If the valve were not effectively balanced, the force needed to close the valve under adverse conditions, such as during a sudden line break, would require a significantly larger spring configuration which would be physically impractical to provide. To insure against valve damage during closure, a dashpot assembly controlled the rate of closure by the springs to insure that it is maintained within an acceptable safe limit.
Although the valve of U.S. Pat. No. 3,888,280 has generally provided a satisfactory means for controlled prevention of fluid flow in either direction, it was found that excessive pulldown forces were generated when a large, 32-inch diameter valve was closed under a particularly demanding flow condition. It might, for example, be necessary to close the valve against fluid flow from below the seat at about 1,000 p.s.i. inlet pressure with the outlet at zero pressure, as might occur with a line rupture. An analysis based on tests of a smaller model and using accepted engineering and analytical methods indicated that the pulldown force generated on such a large valve under these extreme conditions could overload the dashpot and might result in its failure. Although a larger and/or heavier dashpot assembly might have been designed to satisfy this condition, there were other considerations which made this an unattractive alternative. The space provided the dashpot assembly within the operating mechanism for the valve was limited and physically relocating the springs or any other components of the mechanism would require extensive, complicated redesign. Because power plant safety requirements include seismic considerations and the ability of the valve and the pipes to withstand shock, it was also undesirable to increase the mass of the dashpot assembly, and thus the operating mechanism which extends in a cantilevered fashion from the pipes. Therefore, determining the cause of the excessive pulldown forces and providing for their reduction was most desirable.
The rapid fluid flow from beneath the seat produced a velocity pressure which was added to the static pressure of the fluid to produce a higher, stagnation pressure in an axial opening in the main valve disc assembly. The effect of the velocity pressure on the axial opening was then transmitted into the upper chamber of the valve body through the axial opening of the main valve assembly and unintentionally augmented the static pressure needed to provide the desired balanced operation. It was clear that the resulting stagnation pressure acting on top of the main valve disc assembly which created these excessive pulldown forces was not sufficiently opposed by a lesser total pressure from beneath the main disc which acted directly thereon. The lesser total pressure could at least be partially caused by the inclination of the inlet pipe, and thus the direction of fluid flow, with respect to the main disc. Since the stagnation pressure is sufficient to overcome the pressure from beneath the main disc, the main valve disc assembly is positioned downwardly with respect to the stem during closure, preventing the axial opening from being closed by an auxiliary valve disc even though it is capable of closing the axial opening at other times during valve operation.
To prevent the velocity pressure of fluid flow from beneath the seat from producing an excessively over-balanced condition, the valve was improved to include a flow deflecting device to prevent impingement of the fluid flow on an axial opening through the main valve disc assembly which opening facilitated communication of opposite sides of the valve disc assembly. The flow deflecting device preferably included a deflector plate in fixed, spaced relationship from the lower surface of the main disc as is generally disclosed in U.S. patent application Ser. No. 631,286, entitled "Bi-Directional Pressure Balanced Valve," filed on Nov. 12, 1975, by E. B. Pool and L. J. Pavagadhi and assigned to the assignee of the present invention.
However, notwithstanding the improved operation of the valve as disclosed in the above mentioned application, there has been found to exist additional undesired forces acting on the main disc during closure when fluid flow originates from above the main disc. The balanced valve, as mentioned above, is designed for rapid closure in either direction of fluid flow which might result from a break in the line. Similar tests and analyses as those described above have shown that as the valve closes during a condition of loss of pressure in the line below the valve seat, an undesirable upward force on the main disc is generated by the fluid flow. As presently understood, just prior to closure, when the distance of the valve lift from the seat is about 5% to 30% of the minimum port diameter for the valve assembly, a significant resistance to closure is produced by this upward force as the fluid seeks the path of least resistance from the flow passage above the seat, past the disc and seat and into the passage below the seat.
Any resistance to closure must be considered when designing an operating mechanism for the valve. As mentioned hereinabove, such a mechanism might include a spring configuration to provide the force needed for rapid closure. The spring would have to be significantly larger if this resistance to closure could not be reduced or eliminated and a larger spring would tend to complicate space, weight and seismic considerations during design. This would be equally true if other forms of operating mechanisms were employed for rapid closure of the valve. For example, the operating mechanisms disclosed in U.S. patent application Ser. No. 663,787, entitled "Valve Actuator," filed on Mar. 4, 1976, by D. W. Duffey and assigned to the assignee of the present invention, could be utilized with the valve of the present invention. However, as also explained in this application, the size and weight of the mechanism is of real concern so that reduction or elimination of undesired resistance to closure would be highly advantageous when trying to provide an acceptable valve-operating mechanism configuration.